Plastic encapsulation of semiconductor devices by transfer molding is a well known and much used technique. In a typical application, a large number of components or devices are placed in an open multicavity mold with one or more devices in each cavity. For manufacturing convenience, the devices are often coupled, that is, connected together on ladder-like lead frames or substrates. When the mold is closed, the two mold portions, usually called platens or halves, seal against the lead frame or substrate to prevent the border of the frame or substrate from being covered by the injected plastic. The cavities in the mold are connected by a tree-like array of channels or runners connected to a central reservoir from which the molten plastic is fed. Usually, gates, that is, constricted channel regions, are placed at the entrance to each semiconductor device to channel and control the flow and injection velocity of the plastic into the cavity and to permit removal of the finished part which has solidified in the runners.
Typically, powdered or pelletized plastic is placed in a central reservoir and compressed by a ram. The mold and the reservoir are hot. The combination of heat and pressure causes the plastic to liquify and flow through the runners and gates into the individual molding cavities, subsequently encapsulating the semiconductor device and hardening. The mold halves are then separated and the encapsulated parts are removed and trimmed of excess plastic, such as the runners and gates. This removal of the runners and gates can pose a significant problem when molding to lead frames or substrates.
In order to manufacture a quality device, the adhesion of the molding compound to the lead frame or substrate must be maximized. However, this adhesion poses problems when attempting to remove the gate or runner from the substrate. The excellent adhesion of the gate to the substrate causes the substrate to delaminate when the adhesive force between the plastic and the substrate exceeds the cohesive strength of the substrate itself. This creates a defective part which is undesirable. An alternative arrangement of gates or runners that injects the plastic from the top has been used, but this requires a more complex and expensive molding setup.
Thus, a need continues to exist for an improved molding method whereby a simple, two-part mold may be used with conventional gating systems. A gating system employing such a mold that also permits easy release and removal of the molding gate from the substrate, without damaging the substrate, would be highly desired by those in the art.